1. Field of the Invention
The invention relates to the field of light emitting diodes and in particular to a structure and method of increasing the light output.
2. Description of the Prior Art
For years, a significant amount of scientific work has been focused on ways of improving the extraction efficiency of light emitting diodes. Many interesting approaches have been proposed to accomplish this, such as the use of thin light emitting layers with periodic surface texturing, resonant cavities, photon recycling or output coupling through surface plasmons excited at corrugated metal surfaces. External quantum efficiencies of 31% were reported by employing reflection from a bottom metal mirror together with a textured top semiconductor surface.
Apart from efforts to extract as much light as possible from a semiconductor device, it is also possible to enhance the light emission rate within a semiconductor. This approach is based on Purcell""s prediction in 1946 that the radiation rate of an atom placed within a wavelength-sized cavity can be changed. A 5-fold enhancement of spontaneous emission was recently measured in a semiconductor optical microcavity at low temperatures and it has been demonstrated that Purcell factors of about 55 can be achieved when a InGaN/GaN quantum well (QW) is positioned close to a thin silver layer.
In order to build an ideal, highly efficient light-emitting diode (LED), it is desirable to improve the extraction efficiency and simultaneously enhance the spontaneous emission rate. A 15-fold emission intensity enhancement, with Purcell factor Fp=2 was observed in two dimensional periodic thin film photonic crystals. On the other hand, it has been recently discussed that a potentially highly efficient LED consisting of a metal clad dielectric microcavity with periodic texturing of one of metal layers might be possible. This, however, was not actually proved and was only a theoretical conjecture.
What is needed is some kind of a method and structure for enhancing the light emission from LEDs.
The invention is defined as an improvement in a light emitting diode comprising an active light emitting semiconductor layer having a first and second parallel surface, an optically reflecting layer disposed on the first surface of the semiconductor layer, and an optically reflecting grating disposed on the second surface of the active light emitting semiconductor layer. The grating has a pattern which is selected or arranged so that surface plasmon waves are coupled out through the grating. More specifically, in the illustrated embodiment the grating has a regular periodic structure, which is either a one or two dimensionally periodic structure. The optically reflecting grating is partially transparent. In the illustrated embodiment optically reflecting layer disposed on the first surface and the optically reflecting grating disposed on the second surface are conductive layers.
In the illustrated embodiment the active light emitting semiconductor layer comprises a semiconductor membrane, which includes a layer having or acting as a quantum well.
The invention can be characterized as improvement as described above where the grating has a selected pattern which is characterized by its coupling to surface plasmons generated by the light emitting semiconductor layer. The light emitting semiconductor layer in combination with the an optically reflecting layer and the an optically reflecting grating comprise a microcavity. The grating is then characterized by a pattern which couples out surface plasmons in the microcavity.
The invention can also be defined as an improved light emitting diode comprising an active light emitting semiconductor layer having a first and second parallel and opposing surface, an conductive layer disposed on the first surface of the semiconductor layer, and an conductive grating disposed on the second surface of the active light emitting semiconductor layer. The conductive layer and conductive grating with the semiconductor layer therebetween defines a cavity. The grating has a pattern which is selected so that surface plasmon waves within the cavity are coupled through the grating.
Still further the invention is defined as a method of improving emission from a light emitting diode comprising the steps of providing an active light emitting semiconductor layer having a first and second parallel surface. An optically reflecting layer is disposed on the first surface of the semiconductor layer. An optically reflecting grating with a selected pattern is defined therein. The grating is disposed on the second surface of the active light emitting semiconductor layer. The surface plasmon waves are coupled through the optically reflecting grating. The selected pattern is thus characterized being arranged and configured so that surface plasmon waves are coupled therethrough. While the method has been described here for grammatical ease as a combination of steps, it is to be explicitly understood that the method is not necessarily limited by the construction of 35 USC 112 to the disclosed methodology, but includes all acts which fall within the scope of the defining claims.
The invention can be better visualized by now turning to the following drawings where like elements are referenced by like numerals.